1. Field of the Invention
The present disclosure relates generally to an ion source of an ion implanter, and more particularly to an ion source using the inductive magnetic field generated by at least one coil to transmit power into a nearby vessel and using both a first conducting shield around the plasma and a second conducting shield to reduce the oscillation of the plasma inside the vessel and to thereby reduce the energy range of ions extracted from the plasma.
2. Description of Related Art
Ion implantation is a needed but expensive process for modern device fabrication, such as semiconductor device fabrication or flat panel fabrication. It is used mainly to do “doping”—introducing small concentrations of chemically active species into the workpiece, such as semiconducting materials, usually silicon. There is no alternative process to this in most cases. Implantation is also used increasingly for other applications such as definition of critical areas on the device and control of the mobility of dopants in the workpiece. Implantation is expensive because the large and complex implant systems used to perform the process have a high cost and their productivity is not as high as many other fabrication processes. In many cases, this low productivity is due to the limitations of the sources which are used for producing the ions.
The legacy sources now predominantly used in implanters performing this application are the “Bernas” type source—which has a hot cathode to emit electrons at one end of the source. These electrons are confined by a strong magnetic field and reflected at the other end of the source. These electrons form a virtual cathode by traveling along the magnetic field parallel to the extraction slit. Ionization occurs along their path and the ions then fall into the extraction slit for acceleration and ultimately implantation. However, these sources require frequent and expensive maintenance (every 500 hours or so) due to their use of filaments and hot cathodes to produce the electrons. Such hot cathodes in combination with chemistry in the source cause migration of metals, such as tungsten, that ultimately must be removed from the vacuum system and manually cleaned, taking the implanter out of service while the source is rebuilt.
Further, this ion source produces only modest ion currents of the desired species (Boron, arsenic, phosphorus, carbon, silicon, germanium, antimony and others) and, because of residues left in the source after producing one ionic species cannot switch easily from implanting that ionic species to a different one.
A source having such design that it allows the source to be cleaned without manually removing it or breaking vacuum would dramatically reduce the cost, and improve productivity of the very expensive implanter. Further, it would be helpful to the productivity of the implanter if more of the desired species could be produced by the source. This would allow the doping current on the wafer, substrate or workpiece to be increased. Further, if such higher current could be produced at a given level of current extraction from the source—it would both save power of the high voltage accelerating power supply and reduce sputtering of beam stops and baffles where high energy currents of undesired ion species are dumped, reducing particle generation in the vacuum system. Also, the productivity benefits of larger wafers and more uniform and rapid sweeping of the ion beam across the wafer would make it beneficial to have an ion beam extracted from a longer extraction slot. However, this can only be done efficiently if the ion density and extracted current density have only minimal variation along the slot. If the density of plasma along the length is not uniform, the extracted ion beam has focusing that varies along the long transverse dimension of the extracted “ribbon” beam resulting in less efficient utilization of the extracted ions.
Other types of ion sources have been tried and each has been found to have serious problems that ultimately prevented its adoption for mass production. Many designs of the Radio Frequency (rf)/Very High Frequency (VHF) sources have been developed and tested during the past years. In general, a rf/VHF generator is positioned outside a chamber and electrically connected to the coil or antenna. The electromagnetic field from the coil or antenna then transmits the rf/VHF power into the chamber so that the gas inside the chamber is ionized to form a plasma from which an ion beam is extracted through an extraction slit on the front wall of the chamber. Typically, these rf/VHF sources have a higher spread in the energy of the extracted ions due to the rf or VHF oscillation of the plasma potential. Ions extracted at the maximum and minimum of the plasma potential have different energies that may differ by up to ten or more eV resulting in less effective focusing of the ions in the beam, especially at low ion energies and higher currents, as is required for some shallow and ultra-shallow doping processes. When electrostatic shielding is used to reduce the rf plasma potential amplitude in an rf or VHF ion source, there is difficulty in igniting the plasma. Further, such rf sources, whether shielded or not, are hard to clean with in-situ, plasma cleaning unless a biasing electrode is disposed within the plasma vessel. Such electrode within the source may effectively ignite the plasma, but the disadvantages include wear and contamination. Further, these sources usually have difficulty in making the ion density uniform along the length of the extraction slit. This uniformity is needed for efficient ion utilization and cleanliness. Uniformity needs to be maintained for a range of feed gases and source gas pressures without making source volume inordinately large that may make it difficult to fit within the high voltage segment of the beamline within the implanter and inefficient. For examples, some known concepts and variations of the RF/VHF ion source may be provided by the following documents: US 20120049738, US 20120034136, U.S. Pat. No. 6,356,025, U.S. Pat. No. 7,455,030, US 20090032727, US 20100129272, US 20110240876, and US 20110259269.
Accordingly, it is desired to develop a new rf/VHF ion source for providing ion beam with smaller ion energy spread and higher beam uniformity along the ion extraction slit that can generate required ion beams with less hardware cost and higher operation flexibility.